1. Field of the Invention
The present invention relates to numerical displays using plural light sources and more particularly to the energization and control circuits designed for such displays.
2. Description of the Prior Art
Visual displays consisting of characters which are formed by the energization of various point or line element combinations are now quite common. Such alpha-numeric displays include the older incandescent lamp matrix scoreboards and time and temperature displays as well as newer displays using light emitting diodes. One format used frequently is a 5.times.7 rectangular matrix of points (2.sup.35) which has over 34 billion potential characters of which less than 100 are normally useful, assuming both letters and numbers are to be displayed. As digital readouts find more applications and continue to displace analog displays, there occur many instances of strictly numerical fonts which require only eleven characters or less (depending on the need to generate a zero and a blank). Since four binary elements are sufficient to generate sixteen characters (e.g. hexadecimal code), it is quite inefficient to employ seven elements having 128 possibilities merely to generate these few numerals. Consumers demand a familiar font and firmly reject the use of a number system based on 16 instead of 10, so the seven line segment format is particularly popular.
The energization of a seven segment display site can be accomplished by connecting the display element branches in parallel and turning off each element by means of a shunt switch causing current diversion. The current for a site is seven times that of a single segment and it does not vary greatly with the number of segments excited since the current diverted from a segment flows in the shunt switch. (Although series control by current interruption is frequently used, thereby effecting significant dc current reduction, it is normally impractical to provide sufficient smoothing of the ac component for compatibility with either the desired transformerless power supply or a serially connected radio receiver.) Each element conventionally consists of a single light emitting diode creating a single visible line segment. Serial stacking of seven LED diodes (for one character site) with a shunting bipolar transistor on each segment is normally impractical. Since PNP devices would require much more area in an integrated form in order to handle the current for conventional LEDs, NPN devices would be preferred. Assuming NPN devices, the number of LEDs stacked must normally be limited to avoid breaking down the emitter-base junction of the top transistor device when all other segments are "on". Five LEDs at 1.8 volts each stack to 9.0 volts, a voltage which exceeds the maximum for most integrated NPN transistors. Thus, full serial partitioning of one character by stacking seven diodes and using bipolar control transistors is normally precluded. Although a reverse current limiting diode could be employed in conjunction with each NPN, this approach would remain incompatible with single-chip integration in the lowest cost batch fabrication technique currently in widespread use for production of line operated clock/timer ICs: MOS.
If MOSFET control devices are employed with a full serial LED arrangement, there are comparable disadvantages which lead generally to adverse variations in brightness. The saturated MOS drain current is strongly dependent on the gate-to-source voltage, and the latter is difficult to control in the stacked arrangement. The conduction states of segments lower in the stack create a wide dynamic range of source voltages for the upper MOS switches. If brightness is controlled by current amplitude, the problems are further compounded.
In some display applications (e.g. line operated digital clocks and clock radios having LED readouts) a line transformer contributes significantly to the total product cost. An approach which would reduce total display current without sacrificing brightness or introducing an excessive ac current component would allow a smaller, less expensive transformer or eliminate it altogether. In many cases, the transformer could be omitted without increasing the cabinet dissipation beyond acceptable limits.